Update travis section of README

This fixes the example travis config to not ignore install errors for rustfmt, while still not trying to overwrite a previous install.

In addition, this rewrites the script: section to include multiple commands in a way that travis can more accurately indicate which failed and which succeeded.

README.md

@@ -124,13 +124,13 @@ A minimal Travis setup could look like this:
 ```yaml
 language: rust
 cache: cargo
-before_script: (cargo install rustfmt || true)
+before_script:
+- export PATH="$PATH:$HOME/.cargo/bin"
+- which rustfmt || cargo install rustfmt
 script:
-- |
+- cargo fmt -- --write-mode=diff
-  export PATH=$PATH:~/.cargo/bin &&
+- cargo build
-  cargo fmt -- --write-mode=diff &&
+- cargo test
-  cargo build &&
-  cargo test
 ```
 
 Note that using `cache: cargo` is optional but highly recommended to speed up the installation.