Change PATH during travis builds

During the execution of dislocker-find, the loader will call the first
ruby binary in the PATH, which can have a different ruby version against
which dislocker has been build with.
The problem is due to rvm being installed in the ubuntu VM of travis-ci,
but not fully installed.
Thus this commit to try and fix the path from which ruby will be called.

.travis.yml

@@ -8,6 +8,10 @@ os:
     - linux
     - osx
 
+env:
+    global:
+        - PATH="/usr/bin:/usr/local/bin:$PATH"
+
 before_install:
     - if [ "$TRAVIS_OS_NAME" = "linux" ]; then sudo apt-get update -qq; fi
     - if [ "$TRAVIS_OS_NAME" = "linux" ]; then sudo apt-get install -qq libfuse-dev libpolarssl-dev ruby-dev; fi