Check MANIFEST when PR is created

[MichaelTimbert]

Purpose

Run 'make distcheck' in tests.
I followed what was done in "zonemaster/zonemaster-engine#903"
It's the same as "zonemaster/zonemaster-ldns#195"

Context

Fixes #765

How to test this PR

Make sure we don't interfere with the release process:

git clean -dfx   # Start out with a clean repository
perl Makefile.PL # Generate the top-level Makefile
make all         # Generate MO files and modules.txt
make distcheck   # Verify that distcheck doesn't report any missing or unrecognized files

Run the unit tests in the source directory:

make distcheck     # Make sure distcheck doesn't report any missing or unrecognized files
prove t/manifest.t # Verify the new test passes
touch new.file  # Create a file but don't add it to MANIFEST
prove t/manifest.t # Verify the new test fails
echo "new.file" >> MANIFEST.SKIP 
prove t/manifest.t # Verify the new test passes

Run the unit tests in the dist file:

make distcheck                               # Make sure your repo is clean
make dist                                    # Create the dist file
cpanm --test-only Zonemaster-Backend-*.tar.gz # Run the tests included in the dist file

[matsduf]

@MichaelTimbert, please merge.