travis.yml: add protobuf to requirements and exclude plugins

.travis.yml

@@ -2,5 +2,5 @@ sudo: false
 language: python
 python:
     - "2.7"
-install: "pip install slowaes==0.1a1 ecdsa>=0.9 pbkdf2 requests pyasn1 pyasn1-modules tlslite>=0.4.5 qrcode SocksiPy-branch"
-script: nosetests -e gui
+install: "pip install slowaes==0.1a1 ecdsa>=0.9 pbkdf2 requests pyasn1 pyasn1-modules tlslite>=0.4.5 qrcode SocksiPy-branch protobuf"
+script: nosetests -e gui -e plugins