Tweak travis configuration

Trim the amount tested and include a builder for the smallest Rust version

Closes rust-lang#121

.travis.yml

@@ -1,11 +1,22 @@
 language: rust
-rust:
-  - stable
-  - beta
-  - nightly
 sudo: false
-before_script:
-  - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
+
+matrix:
+  include:
+    - rust: 1.21.0
+    - rust: stable
+    - os: osx
+    - rust: beta
+    - rust: nightly
+
+    - rust: nightly
+      before_script:
+        - pip install 'travis-cargo<0.2' --user && export PATH=$HOME/.local/bin:$PATH
+      script:
+        - cargo doc --no-deps --all-features
+      after_success:
+        - travis-cargo --only nightly doc-upload
+
 script:
   - export CARGO_TARGET_DIR=`pwd`/target
   - cargo build --verbose
@@ -20,15 +31,11 @@ script:
   - cargo clean && cargo build
   - cargo doc --no-deps
   - cargo doc --no-deps --manifest-path=miniz-sys/Cargo.toml
-after_success:
-  - travis-cargo --only nightly doc-upload
+
 env:
   global:
     secure: "PHVT7IaeP5nQQVwGHKwqCYBDp0QyetSlER7se2j2Xgfx+lw3Bu6VWH6VF04B636Gb0tHPN/sUCXSgGRcvDuy6XFOev4LfynoYxNKgHJYg2E34EP2QLwsFfnvE4iujaG3GJk3o935Y7OYGv2OP1HeG4Mv6JhQK0GLnNDBZQ65kWI="
 
 notifications:
   email:
     on_success: never
-os:
-  - linux
-  - osx