travis/coverity: add coverity support

Run only on push to branch coverity-scan. We can use a cron script to do
this 4 times a week (that's our allowance).

NOTE: possible future improvements are:

1. Fold the build matrix item into another short one so we don't overburden
   travis. It's a little less clear but it should be nicer on the
   infrastructure.
2. Change the security token, one can do that from the coverity admin page.
3. Don't do the naive `make depend`, but use the prebuilt libraries.

.travis.yml

@@ -1,9 +1,15 @@
 language: c
 env:
-  - TRAVIS_BUILD_TYPE=clang/asan
-  - TRAVIS_BUILD_TYPE=gcc/ia32
-  - TRAVIS_BUILD_TYPE=gcc/unittest
-  - TRAVIS_BUILD_TYPE=clint
-  - TRAVIS_BUILD_TYPE=api/python
+  global:
+    # The next declaration is the encrypted COVERITY_SCAN_TOKEN, created
+    # via the "travis encrypt" command using the project repo's public key
+    - secure: "QEz92NyItkzQu52kCFD928jEwUYnA2OIgSyeNrp+Y3gm5rOmSZerY8hGiXyNZxocap9+qIPCapRRYU3ZYKWZPeucWMLN3aIjxAFdhugKbnmNYE1jFugb6b8N3SxiX/3206NHXlYaz0OZhh6OBAFmPUXamJC8OrWVgPNPo7wv4UQ="
+  matrix:
+    - TRAVIS_BUILD_TYPE=clang/asan
+    - TRAVIS_BUILD_TYPE=gcc/ia32
+    - TRAVIS_BUILD_TYPE=gcc/unittest
+    - TRAVIS_BUILD_TYPE=clint
+    - TRAVIS_BUILD_TYPE=api/python
+    - TRAVIS_BUILD_TYPE=coverity
 script:
   - ./scripts/travis.sh

scripts/travis.sh

@@ -77,7 +77,16 @@ fi
 # for more information.
 MAKE_CMD="make -j2"
 
-if [ "$TRAVIS_BUILD_TYPE" = "clang/asan" ]; then
+if [ "$TRAVIS_BUILD_TYPE" = "coverity" ]; then
+    curl -s https://scan.coverity.com/scripts/travisci_build_coverity_scan.sh |
+        COVERITY_SCAN_PROJECT_NAME="neovim/neovim" \
+        COVERITY_SCAN_NOTIFICATION_EMAIL="coverity@aktau.be" \
+        COVERITY_SCAN_BRANCH_PATTERN="coverity-scan" \
+        COVERITY_SCAN_BUILD_COMMAND_PREPEND="$MAKE_CMD deps" \
+        COVERITY_SCAN_BUILD_COMMAND="$MAKE_CMD nvim" \
+        bash
+    exit $?
+elif [ "$TRAVIS_BUILD_TYPE" = "clang/asan" ]; then
 	if [ ! -d /usr/local/clang-3.4 ]; then
 		echo "Downloading clang 3.4..."
 		sudo sh <<- "EOF"