Problem: Travis CI building fails (fixes #2226)

Solution:
- added a step install a newer protoc

.travis.yml

@@ -35,7 +35,6 @@ rust: &rust
         - binutils-dev
         - libc6-dev
         - libudev-dev
-        - protobuf-compiler
   cache:
     directories:
       - $HOME/.cargo
@@ -58,6 +57,7 @@ rust: &rust
     - TDBE_MRENCLAVE=0000000000000000000000000000000000000000000000000000000000000000
     - NETWORK_ID=ab
   before_install: # versions from https://github.com/erickt/rust-zmq/blob/master/.travis.yml
+    - ./ci-scripts/install_protoc.sh
     - |
       if [[ "$TRAVIS_RUST_VERSION" != nightly ]]; then
         sed -i.bak -E "s/default = \[\".+\"\]/default = \[\"mock-enclave\"\]/" chain-abci/Cargo.toml;

ci-scripts/install_protoc.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+PROTOBUF_VERSION=3.3.0
+PROTOC_FILENAME=protoc-${PROTOBUF_VERSION}-linux-x86_64.zip
+pushd /home/travis
+wget https://github.com/google/protobuf/releases/download/v${PROTOBUF_VERSION}/${PROTOC_FILENAME}
+unzip ${PROTOC_FILENAME}
+bin/protoc --version
+popd