specify stricter constraint for libc version

sass-sys specifies a constraint of "0.1", but sass-rs specifies "0",
which allows version 0.2 and above

this shouldn't be a problem, because a "0.1" version would satisfy both
constraints, but I think the reason this isn't working is because of
rust-lang/cargo#2064 which might not get resolved anytime soon

without this fix, building sass-rs fails because it tries to use
libc 0.2 which has a different interface from the 0.1 that sass-sys used

this gets things building again for now, but we should look into
updating to the 0.2 interface when we get a chance

Cargo.toml

@@ -17,4 +17,4 @@ path = "src/lib.rs"
 
 [dependencies]
 sass-sys = "0"
-libc = "0"
+libc = "0.1"