getLabelCache function

added access to the drawn label cache when using mapserver 7.
use tbonfort's branch of ms 7 for travis
fix travis env
use cmake
better travis setup
reduce deps
fix install path
add missing include for travis
remove unused require
configure Release build for travis
add labelcache test

check that the label cache is retrieved and has the right labels.
add hashtable support

added support for hashtableObj to provide accessors for map and layer
level metadata.
update travis config for mapserver / master configuration, update test output to match this version.
added layer::connection accessor
add layer scale denom accessors
add property setter for hashtable object and tests for map and layer metadata setting metadata
make label cache method compatible with MS 6.4 and 6.5