Fix Travis builds

The maxavro requires some special libraries which aren't always
available. For this reason, the maxavro library shouldn't be build unless
all libraries are available.

.travis/build_maxscale.sh

@@ -12,9 +12,9 @@ echo TRAVIS_BUILD_DIR: ${TRAVIS_BUILD_DIR}
 mkdir build
 cd build
 
-cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_TESTS=Y
+cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_TESTS=Y -DBUILD_AVRO=N
 
-make VERBOSE=1
+make
 make test
 sudo make install