requirements: Start using fixed versions

requirements: Start using fixed versions

Given that 'tox' doesn't actually read any of these, there's no reason
to use ranges of requirements. Instead, use the latest and greatest for
live instances and rely on tox to validate behavior with older versions.

The selenium dependency, which is no longer required since commit
bab2895f, is removed. The psycopg2 dependency is updated to use
psycopg2-binary, as this avoids the need for the libpg library and
removes a deprecation warning.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Signed-off-by: Daniel Axtens <dja@axtens.net>