Improve Fish version check; don't exit on failure
Improve Fish version check; don't exit on failure

Fish's version number in its master branch has a hyphen-suffixed
addendum that causes incorrect version comparison results, so now that
suffix is stripped before comparing to the minimum version number.

If that check fails, VirtualFish now shows a warning and continues
instead of exiting with an error, in an effort to more gracefully handle
situations where the version check fails even though the minimum Fish
version requirement is met. This may cause problems if the check results
are actually correct and folks ignore the warning, since subsequent
VirtualFish invocations such as `vf new` may yield expected results on
older versions of Fish.

Fixes #177

adambrenecki/virtualfish,adambrenecki/virtualfish