This fixes the build against ZMQ 3.2. It also make Travis test the build against both 4.x and 3.x version
Merge pull request #63 from xaqq/fix_build_3.2

This fixes the build against ZMQ 3.2.
Fix compilation with gcc < 4.4

Older version of gcc does not support "enum class" declaration. However,
there is already a macro to fix this compatibility issue:
ZMQPP_COMPARABLE_ENUM. This patch use this macro instead of "enum class"
Merge pull request #64 from jerome-pouiller/develop

Fix compilation with gcc < 4.4
Merge branch 'develop' of https://github.com/zeromq/zmqpp into develop