process: Update min supported rust version as per the Tokio project policy

.travis.yml

@@ -3,7 +3,14 @@ sudo: false
 
 matrix:
   include:
-    - rust: 1.26.0
+    # This represents the minimum Rust version supported by
+    # Tokio. Updating this should be done in a dedicated PR and
+    # cannot be greater than two 0.x releases prior to the
+    # current stable.
+    #
+    # Tests are not run as tests may require newer versions of
+    # rust.
+    - rust: 1.33.0
     - rust: stable
     - os: osx
     - rust: beta