Get Travis to check if the version number matches the tag number

[gemmaleigh]

If it does, don't update the latest-release branch.
Otherwise, if the version number has changed - update the latest release branch.

[gemmaleigh]

If you'd like to test this, I commented out lines 10 & 11 and 28 to 30.

This can then be run locally:

./update-release-branch.sh

The output is:

++ cat VERSION.txt
+ VERSION_TAG=4.0.0
+ git rev-parse 4.0.0
+ echo 'Not updating the latest-release branch as the tag already exists...'
Not updating the latest-release branch as the tag already exists...

cc. @robinwhittleton @joelanman

Lines 10 & 11 check:

• we want this to only happen from our repo, not forks
• check this is not a pull request
• check we're on the master branch