Update Travis Docker file for Trusty to build gRPC code (#709)

[RFC=0002](https://github.com/googlecartographer/rfcs/blob/master/text/0002-cloud-based-mapping-1.md)

Dockerfile.trusty

@@ -19,5 +19,7 @@ COPY scripts/install_ceres.sh cartographer/scripts/
 RUN cartographer/scripts/install_ceres.sh && rm -rf ceres-solver
 COPY scripts/install_proto3.sh cartographer/scripts/
 RUN cartographer/scripts/install_proto3.sh && rm -rf protobuf
+COPY scripts/install_grpc.sh cartographer/scripts/
+RUN cartographer/scripts/install_grpc.sh && rm -rf grpc
 COPY . cartographer
-RUN cartographer/scripts/install_cartographer.sh && rm -rf cartographer
+RUN cartographer/scripts/install_cartographer_with_grpc.sh && rm -rf cartographer

scripts/install_cartographer_with_grpc.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+# Copyright 2017 The Cartographer Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o verbose
+
+# Build and install Cartographer.
+cd cartographer
+mkdir build
+cd build
+cmake .. -DBUILD_GRPC=ON -G Ninja
+ninja
+ninja test
+sudo ninja install

scripts/install_grpc.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+# Copyright 2017 The Cartographer Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -o errexit
+set -o verbose
+
+VERSION="v1.7.2"
+
+# Build and install gRPC.
+git clone -b ${VERSION} https://github.com/grpc/grpc
+cd grpc
+git submodule update --init
+make
+sudo make install

scripts/install_proto3.sh

@@ -26,6 +26,7 @@ git checkout tags/${VERSION}
 mkdir build
 cd build
 cmake -G Ninja \
+  -DCMAKE_POSITION_INDEPENDENT_CODE=ON \
   -DCMAKE_BUILD_TYPE=Release \
   -Dprotobuf_BUILD_TESTS=OFF \
   ../cmake