Speedup Travis CI builds by non-optimizing compiles

Especially lxml takes very long to install. By specifying the `-O0`
compiler flag this is reduced by more than half.

The actual unit tests are slightly slower with the non-optimized
C modules, but we have a netto win.

.travis.yml

@@ -7,7 +7,7 @@ services: redis-server
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -y swig
-  - pip install -r requirements.txt
+  - CFLAGS=-O0 pip install -r requirements.txt
   - pip install psycopg2
 install: pip install .
 before_script: