Remove ~ dependency on clap.

The point of the ~ dependency was to avoid implicitly increasing the
minimum Rust version required to compile ripgrep. However, clap's policy
is to support at least two prior releases of Rust (which roughly
corresponds to the convention that others use too), and that is probably
good enough.

The problem with using a ~ dependency is that it can make packaging
ripgrep in Linux distros difficult, because it means the packager may be
forced to package multiple compatible versions of the same library.

Fixes #271

Cargo.toml

@@ -26,7 +26,7 @@ path = "tests/tests.rs"
 
 [dependencies]
 bytecount = "0.1.4"
-clap = "~2.19.0"
+clap = "2.19.0"
 ctrlc = "2.0"
 env_logger = "0.3"
 grep = { version = "0.1.4", path = "grep" }