Issue #5; up minimum Comnipay version (~3.0-alpha); remove pre-7.1 travis tests
Updates for:
- Issue #5, change root namespace from Academe to Omnipay
- Minimum PHP version now 7,1
- Minimum Omnipay Common version now ~3.0-alpha.
- Remove pre-7.1 travis tests. Now testing against PHP 7.1 and 7.2