travis: also test with minimum supported Rust version

The 1.2.0 release increased the minimum supported version of Rust from
below 1.11.0 to 1.13.0 due to the introduction of code that use the
"?" operator. This caused unexpected build failures in dependent
crates, such as clap.

This PR updates Travis to also test against the current minimum
version. Changes to this version will thus fail PRs and it will become
an active decision to bump the minimum supported Rust version.

Fixes #26.

Author: mgeisler

.travis.yml

@@ -1,12 +1,15 @@
 language: rust
-rust: stable
+rust:
+  - stable
+  - 1.13.0
 os: linux
 script:
   - cargo build --verbose
   - cargo test --verbose
   - rustdoc --test README.md -L target/debug -L target/debug/deps
   - cargo doc
 after_success: |
+  [ $TRAVIS_RUST_VERSION = stable ] &&
   [ $TRAVIS_BRANCH = master ] &&
   [ $TRAVIS_PULL_REQUEST = false ] &&
   echo '<meta http-equiv=refresh content=0;url=unicode_segmentation/index.html>' > target/doc/index.html &&