introduce install step to .travis.yml and cache pip installs

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>

.travis.yml

@@ -2,6 +2,7 @@ language: python
 services:
     - docker
 
+cache: pip
 env:
     global:
         - TEST_RUNNER_IMAGE="martbab/freeipa-fedora-test-runner:master-latest"
@@ -20,7 +21,7 @@ env:
             test_ipaserver
             test_pkcs10
             test_xmlrpc/test_[l-z]*.py"
-before_install:
+install:
     - pip install pep8
     - >
       pip3 install