Hook repo to Travis-CI

From now on, all pushes to master will result in tmux compiling against a
linux-based distribution (Debian).  This will make it easier for automatic
merges between OpenBSD and portable to be tested, without the need for so
much manual syncing.

Any build failures will be reported to me, and fixed accordingly.

.travis.yml

@@ -0,0 +1,10 @@
+language: c
+matrix:
+    include:
+        - compiler: gcc
+        - compiler: clang
+          env: CFLAGS="-g -O2"
+before_install:
+    - sudo apt-get update -qq
+    - sudo apt-get -y install debhelper autotools-dev dh-autoreconf file libncurses5-dev libevent-dev pkg-config libutempter-dev build-essential
+script: (CFLAGS= ./autogen.sh) && configure --enable-debug && make