skip pyinstaller on source only test linux builds

src/travis/linux-before-install.sh

@@ -104,10 +104,11 @@ else
     $pip install staticx
   fi
 
+  $pip install --upgrade https://github.com/pyinstaller/pyinstaller/archive/develop.tar.gz
+
   cd $whereibelong
 fi
 
 echo "Upgrading pip packages..."
 $pip list --outdated --format=freeze | grep -v '^\-e' | cut -d = -f 1  | xargs -n1 $pip install -U
 $pip install --upgrade -r src/requirements.txt
-$pip install --upgrade https://github.com/pyinstaller/pyinstaller/archive/develop.tar.gz