Fix renamed is_valid() method call

[aricooperdavis]

• Describe the scope of your change--i.e. what the change does and what parts
  of the code were modified.

This change fixes a bug with the code following a change to the semver package which renames the isvalid() method to is_valid(). Fixes #193

• Describe any known limitations with your change.

n/a

• Please run any tests or examples that can exercise your modified code.

n/a

[dhalbert]

Thanks very much!