Update guide.md to use "language: rust" in .travis.yml

travis-ci now supports "language: rust", which eliminates the need to call rustup.sh in .travis.yml. See http://www.reddit.com/r/rust/comments/2ecocp/travis_ci_supports_rust/ for details.
auto merge of #428 : garrison/cargo/patch-3, r=alexcrichton

travis-ci now supports "language: rust", which eliminates the need to call rustup.sh in .travis.yml. See http://www.reddit.com/r/rust/comments/2ecocp/travis_ci_supports_rust/ for details.

This is the same change as #425, but on the master branch in guide.md, not guide.html.
Use `mod` where possible

Example:

```
use std::fmt;
use std::fmt::Show;
```

becomes

```
use std::fmt::{mod, Show};
```
auto merge of #430 : aochagavia/cargo/mod, r=alexcrichton

Example:

```
use std::fmt;
use std::fmt::Show;
```

becomes

```
use std::fmt::{mod, Show};
```
Add a new CARGO_MANIFEST_DIR environment variable

All subprocesses will now be invoked with CARGO_MANIFEST_DIR pointing at the
root of the source directory that they are working on (compiling). This commit
also reorganizes the version environment variables to use the new
infrastructure.

Closes #433
auto merge of #434 : alexcrichton/cargo/issue-433, r=huonw

All subprocesses will now be invoked with CARGO_MANIFEST_DIR pointing at the root of the
source directory that they are working on (compiling). This commit also
reorganizes the version environment variables to use the new infrastructure.

Closes #433
Fix building with current rust
Print out the rustc version before each build
Fix a flaky test while the compiler is changing
auto merge of #450 : alexcrichton/cargo/fix-cargo, r=alexcrichton
Add a --no-run option to `cargo test`

This allows tests to be built, but not run.