Travis should test mandatory dependencies

- travis script tests mandatory dependencies
- may require additional commits as I don't know exactly how travis handles
  errors from multiple scripts

.travis.yml

@@ -8,4 +8,6 @@ addons:
       - libcap-dev
 install:
   - python setup.py install
-script: pybitmessage -t
+script:
+  - python checkdeps.py
+  - pybitmessage -t

checkdeps.py

@@ -11,6 +11,7 @@ Limitations:
 """
 
 import os
+import sys
 from distutils.errors import CompileError
 try:
     from setuptools.dist import Distribution
@@ -161,5 +162,7 @@ if (not compiler or prereqs) and OPSYS in PACKAGE_MANAGER:
     if not compiler:
         compilerToPackages()
     prereqToPackages()
+    if mandatory:
+        sys.exit(1)
 else:
     print("All the dependencies satisfied, you can install PyBitmessage")