Fix types in test_cmemcached
Fix types in test_cmemcached

The memcached append and prepend operations require byte string
values, so unconditionally use the 'b' prefix on Python 2 and 3.

This is valid syntax in Python 2.6+, which matches the supported
versions in .travis.yml.

lericson/pylibmc,lericson/pylibmc,lericson/pylibmc