Travis: Install pokedex with pip install -e .

I would explain why, but this commit will eventually be squashed into the other
ones so there's no point.

(Short explanation: without -e, the database location defaults to
$ENV/lib/python2.7/site-packages/pokedex/data/pokedex.sqlite when running the
pokedex command, but when running py.test it defaulted to
./pokedex/data/pokedex.sqlite, which obviously caused problems.)

.travis.yml

@@ -4,6 +4,6 @@ python:
     - "3.4"
     - "pypy"
 
-install: pip install .
+install: pip install -e .
 before_script: pokedex setup -v
 script: py.test