Fix bump command to show bumped version for tag & release messages

[PikachuEXE]

#79 related but incomplete fix

After patch:

Before patch:

To use correct file name for built gem in pretend mode (it's already correct in actual run)
A big change seems to be required since the file name is read through:

• https://github.com/svenfuchs/gem-release/blob/master/lib/gem/release/cmds/release.rb#L98
• https://github.com/svenfuchs/gem-release/blob/master/lib/gem/release/context/gem.rb#L19
• https://github.com/svenfuchs/gem-release/blob/master/lib/gem/release/context/gemspec.rb#L21

And the scale of change required to fix a small pretend mode only issue seems too large for me
Also the filename doesn't matter anyway (Let me know if I am wrong)
If you have better idea let me know (or just submit a PR)

[coveralls]

Coverage increased (+0.01%) to 96.423% when pulling 40ef67f on fix/post-bump-message into f5776ff on master.

[coveralls]

Coverage increased (+0.01%) to 96.423% when pulling 40ef67f on fix/post-bump-message into f5776ff on master.