Upgrade to Gradle 5.3.1 and fix signing issue

[ob-stripe]

r? @mickjermsurawong-stripe
cc @stripe/api-libraries @hans-stripe

This fixes the signature issue we ran into yesterday. The problem is described in this comment:

The com.vanniktech.maven.publish unconditionally adds the jar task to configurations.archives.artifacts where it has already been added by the java plugin thus causing the duplicate.

We're not using the com.vanniktech.maven.publish plugin, but we had the same issue: we manually added the jar task to configurations.archives.artifacts.

After removing the jar task from configurations.archives.artifacts, I've verified that things work as expected:

$ GNUPGHOME="/path/to/gnupg" GRADLE_OPTS="-Dorg.gradle.project.signing.gnupg.keyName=C33033E4B583FE612EDE877C05D02D3D57ABFF46" ./gradlew clean sign
...
BUILD SUCCESSFUL in 50s
8 actionable tasks: 7 executed, 1 up-to-date

$ ls -l build/libs
total 15848
-rw-r--r--  1 ob  staff  5201573 Apr 10 10:31 stripe-java-9.0.0-javadoc.jar
-rw-r--r--  1 ob  staff      508 Apr 10 10:31 stripe-java-9.0.0-javadoc.jar.asc
-rw-r--r--  1 ob  staff   795820 Apr 10 10:31 stripe-java-9.0.0-sources.jar
-rw-r--r--  1 ob  staff      508 Apr 10 10:31 stripe-java-9.0.0-sources.jar.asc
-rw-r--r--  1 ob  staff  1763670 Apr 10 10:30 stripe-java-9.0.0.jar
-rw-r--r--  1 ob  staff      508 Apr 10 10:31 stripe-java-9.0.0.jar.asc

$ gpg --verify build/libs/stripe-java-9.0.0.jar.asc build/libs/stripe-java-9.0.0.jar
gpg: Signature made Wed Apr 10 10:31:27 2019 PDT
gpg:                using RSA key C33033E4B583FE612EDE877C05D02D3D57ABFF46
gpg: Good signature from "Stripe <[email protected]>" [expired]
gpg:                 aka "Stripe <[email protected]>" [expired]
gpg: Note: This key has expired!
Primary key fingerprint: C330 33E4 B583 FE61 2EDE  877C 05D0 2D3D 57AB FF46

$ gpg --verify build/libs/stripe-java-9.0.0-sources.jar.asc build/libs/stripe-java-9.0.0-sources.jar
gpg: Signature made Wed Apr 10 10:31:27 2019 PDT
gpg:                using RSA key C33033E4B583FE612EDE877C05D02D3D57ABFF46
gpg: Good signature from "Stripe <[email protected]>" [expired]
gpg:                 aka "Stripe <[email protected]>" [expired]
gpg: Note: This key has expired!
Primary key fingerprint: C330 33E4 B583 FE61 2EDE  877C 05D0 2D3D 57AB FF46

$ gpg --verify build/libs/stripe-java-9.0.0-javadoc.jar.asc build/libs/stripe-java-9.0.0-javadoc.jar
gpg: Signature made Wed Apr 10 10:31:27 2019 PDT
gpg:                using RSA key C33033E4B583FE612EDE877C05D02D3D57ABFF46
gpg: Good signature from "Stripe <[email protected]>" [expired]
gpg:                 aka "Stripe <[email protected]>" [expired]
gpg: Note: This key has expired!
Primary key fingerprint: C330 33E4 B583 FE61 2EDE  877C 05D0 2D3D 57AB FF46

Fixes #733.

[mickjermsurawong-stripe]

Thank you ob for digging further into this!
LGTM

[manosbatsis]

This helped fix my (irrelevant) build guys, thanks a bunch