Relax absolute pin on protobuf.

We should allow versions later than the one we know is 'least acceptable'.

tox.ini

@@ -10,7 +10,7 @@ commands =
 deps =
     nose
     unittest2
-    protobuf==3.0.0-alpha-1
+    protobuf>=3.0.0-alpha-1
 
 [testenv:cover]
 basepython =
@@ -20,7 +20,7 @@ commands =
 deps =
     nose
     unittest2
-    protobuf==3.0.0-alpha-1
+    protobuf>=3.0.0-alpha-1
     coverage
     nosexcover
 
@@ -56,7 +56,7 @@ deps =
     pep8
     pylint
     unittest2
-    protobuf==3.0.0-alpha-1
+    protobuf>=3.0.0-alpha-1
 
 [testenv:regression]
 basepython =
@@ -65,7 +65,7 @@ commands =
     {toxinidir}/scripts/run_regression.sh
 deps =
     unittest2
-    protobuf==3.0.0-alpha-1
+    protobuf>=3.0.0-alpha-1
 
 [testenv:regression3]
 basepython =
@@ -77,4 +77,4 @@ deps =
 #   Use a development checkout of oauth2client until a release is made
 #   which fixes https://github.com/google/oauth2client/issues/125
     -egit+https://github.com/google/oauth2client.git#egg=oauth2client
-    protobuf==3.0.0-alpha-1
+    protobuf>=3.0.0-alpha-1