Fix links in RELEASES.md for 1.10.0 through 1.12.0

Many links in this series have the `[link text]` and `(url)` on separate
lines, which doesn't get correctly interpreted in markdown.  Or maybe it
once did, but it doesn't now.  This patch joins the lines together.

Here is the content rendered [before](https://github.com/rust-lang/rust/blob/2971d491b955fd8bb504c8ca675b8116b8b1550e/RELEASES.md#version-1120-2016-09-29) and [after](https://github.com/cuviper/rust/blob/e8c4b7af216c9e035b40f78cbd77a35614fdc240/RELEASES.md#version-1120-2016-09-29).