travis: use env test matrix for ZDOTDIR

.travis.yml

@@ -1,10 +1,14 @@
 language: python
 python:
   - 3.4
-before_script:
-  - sudo apt-get install zsh
+env:
+  ZDOTDIR=tests/ZDOTDIR
+  ZDOTDIR=tests/ZDOTDIR.clobber
+  ZDOTDIR=tests/ZDOTDIR.invalid-module_path
+  ZDOTDIR=tests/ZDOTDIR.loadviafunction
 install:
-  - sudo pip install cram
+  - sudo apt-get install zsh
+  - pip install cram
 script:
   - zsh --version
-  - make test_full
+  - make test