cargo: restrict unicode-segmentation to ~1.1.0

The newly released version 1.2.0 of unicode-segmentation adds code
that use the "?" operator, which in turn requires Rust 1.13.0.
However, clap currently still works with Rust 1.11.0 and this caused
build failures:

  https://travis-ci.org/kbknapp/clap-rs/jobs/235010822

The changes since 1.1.0 seem to be related cursors/iterators and I
think clap can work fine without them.

Cargo.toml

@@ -19,7 +19,7 @@ A simple to use, efficient, and full featured  Command Line Argument Parser
 bitflags              = "0.8.0"
 vec_map               = "0.8"
 unicode-width         = "0.1.4"
-unicode-segmentation  = "1.0.1"
+unicode-segmentation  = "~1.1.0" # 1.2.0 requires Rust 1.13.0
 strsim    = { version = "0.6.0",  optional = true }
 ansi_term = { version = "0.9.0",  optional = true }
 term_size = { version = "0.3.0",  optional = true }