ci: fix java 7 build

In order to prevent:

> Error: Exception in thread "main" java.lang.UnsupportedClassVersionError: com/fasterxml/jackson/databind/ObjectMapper : Unsupported major.minor version 52.0
>	at java.lang.ClassLoader.defineClass1(Native Method)
>	at java.lang.ClassLoader.defineClass(ClassLoader.java:808)

Author: darrachequesne

Diff for: .github/workflows/ci.yml

@@ -11,6 +11,7 @@ jobs:
     runs-on: ubuntu-latest
 
     strategy:
+      fail-fast: false
       matrix:
         java: [7, 8, 11]
 

Diff for: pom.xml

@@ -184,7 +184,7 @@
       <plugin>
         <groupId>org.sonatype.plugins</groupId>
         <artifactId>nexus-staging-maven-plugin</artifactId>
-        <version>1.6.13</version>
+        <version>1.6.7</version>
         <extensions>true</extensions>
         <configuration>
           <serverId>ossrh</serverId>