Change validation function (valid-format?) to boolean test.

Prior to this commit, (valid-format?) threw an
InvalidArgumentException when run on an invalid semver.
This commit changes this validation test to return false in
this case and changes the corresponding (parse) unit tests
to check for AssertionErrors.
Merge pull request #3 from jhcarr/maint/conform-validation-to-boolean-format

Change validation function (valid-format?) to boolean test.