Bump min Rust version to 1.33.0

jpeg-decoder relies on matching all of 0x00..=0xff exhausts u8,
which requires 1.33.0.

.travis.yml

@@ -15,7 +15,7 @@ addons:
 matrix:
     include:
         - os: linux
-          rust: 1.31.1
+          rust: 1.33.0
         - os: linux
           rust: stable
         - os: osx
@@ -37,7 +37,7 @@ script:
 
 after_success:
     - >
-        if [ "$TRAVIS_OS_NAME" = 'linux' ]; then 
+        if [ "$TRAVIS_OS_NAME" = 'linux' ]; then
             cargo install cargo-kcov &&
             ( cargo kcov --print-install-kcov-sh | bash ) &&
             cargo kcov --coveralls