Correct writing of hyphenated and underscored package names

[caphrim007]

This patch changes hashin such that,

If the package specified has underscores in its name, but the new package has hyphens, the following occurs

• If there is a new version, hashin updates the package in requirements to be the hyphenated name
• If there is no new version, the underscored name remains in place

Prior to this patch, the 1st case about would not occur. Instead, hashin would append the new version in its hyphenated form to the requirements file and leave the old version in its underscored form in the requirements file.

[caphrim007]

@peterbe ready for review

[caphrim007]

Fixes #116

[caphrim007]

@peterbe I will send updates shortly. thanks for reviewing!

[caphrim007]

changes made. Ready for review

[peterbe]

Travis isn't reporting checks here on the PR, but: https://travis-ci.org/github/peterbe/hashin/builds/698623654 is all green!