move to go modules and upgrade bblfsh client version

[erizocosmico]

Closes #838
Fixes #852
Fixes #846

Updated rev-upgrade tool to work with go modules.
Upgraded go-mysql-server that does not use gopkg-in anymore.
Upgraded bblfsh client to use the new one that uses go modules.

• I updated the documentation explaining the new behavior if any.
• I updated CHANGELOG.md file adding the new feature or bug fix.
• I updated go-mysql-server using make upgrade command if applicable.
• I added or updated examples if applicable.
• I checked that changes on schema are reflected into the documentation, if applicable.

[kuba--]

I don't know if there is any other option than blindly accept it ;)

[kuba--]

Fix tests

fixtures folder not found

[erizocosmico]

Ugh, yes. It happened to me locally, then it didn't anymore and forgot about that. go-git-fixtures does not work with go modules * sobs in spanish *

[juanjux]

I trust @erizocosmico and his sed-foo prowess completely so I hereby approve the five thousand files of changes.

[erizocosmico]

Waiting for a PR to be merged on go-git-fixtures before updating here and make tests work.

[ajnavarro]

@erizocosmico go-git-fixtures is already merged, shall we continue with this?

[erizocosmico]

Updated

[erizocosmico]

All fixed. Ready to be merged