Enable RE2 testing with Travis

Ubuntu 14.04 doesn't have RE2 package altogether,
and Ubuntu 16.04 RE2 package is too old.

Ubuntu 18.04 RE2 package might work, but this Ubuntu verison it's not
supported by Travis yet.

So build RE2 from source.

Author: WGH-

.travis.yml

@@ -30,6 +30,7 @@ env:
   - OPTS="--without-ssdeep $OPTS"
   - OPTS="--without-lua $OPTS"
   - OPTS="--without-maxmind $OPTS"
+  - OPTS="--without-re2 $OPTS"
 
 before_script:
   - echo $TRAVIS_OS_NAME
@@ -41,6 +42,7 @@ before_script:
   - '[ "$TRAVIS_OS_NAME" != linux ] || sudo apt-get update'
   - '[ "$TRAVIS_OS_NAME" != linux ] || sudo apt-cache search maxmind'
   - '[ "$TRAVIS_OS_NAME" != linux ] || sudo apt-get install -y libmaxminddb-dev'
+  - '[ "$OPTS" == "*--without-re2*" ] || ./.travis/install_re2.sh'
 
 script:
   - ./build.sh

.travis/install_re2.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+# Ubuntu 14.04 doesn't have RE2 package altogether,
+# and Ubuntu 16.04 RE2 package is too old.
+#
+# Ubuntu 18.04 RE2 package might work, but this Ubuntu verison it's not
+# supported by Travis yet.
+
+RELEASE=2019-01-01
+
+set -ex
+
+cd /tmp
+wget https://github.com/google/re2/archive/$RELEASE.tar.gz -O re2.tar.gz
+tar -xf re2.tar.gz
+(cd re2-$RELEASE && make && sudo make install)