Require exact version of protobuf (2.10.0)

The version of compiled files needs to match the protobuf version,
including patch level.

Cargo.toml

 system-libz = ["flate2"]
 
 [dependencies]
-protobuf = "~2.10"
+protobuf = "=2.10.0"
 byteorder = "1.1"
 flate2 = { version = "1.0", optional = true }
 inflate = "0.4"