Improve Travis reliability

Previously, we were using too many parallel instances of $CXX, which caused
crashes. Now we use -j 2, which seems to work. Also, use fewer processes during
testing.

.travis.yml

-language: dart
+language: cpp
 sudo: required
 before_install:
   - ./travis/before_install.sh

sky/tools/gn

@@ -29,7 +29,7 @@ def to_gn_args(args):
     gn_args = {}
 
     gn_args["is_debug"] = args.debug
-    gn_args["is_clang"] = args.target_os not in ['android']
+    gn_args["is_clang"] = args.clang and args.target_os not in ['android']
 
     if args.target_os == 'android':
         gn_args["target_os"] = "android"
@@ -79,6 +79,9 @@ def main():
   parser.add_argument('--goma', default=True, action='store_true')
   parser.add_argument('--no-goma', dest='goma', action='store_false')
 
+  parser.add_argument('--clang', default=True, action='store_true')
+  parser.add_argument('--no-clang', dest='clang', action='store_false')
+
   args = parser.parse_args()
 
   command = ['gn', 'gen', '--check']

travis/build.sh

@@ -3,5 +3,5 @@ set -ex
 
 # Linux Debug
 ./sky/tools/gn --debug
-ninja -j 8 -C out/Debug
-./sky/tools/test_sky --debug
+ninja -j 2 -C out/Debug
+./sky/tools/test_sky --debug --child-processes=1

travis/gclient

@@ -5,4 +5,4 @@ solutions = [{
   "managed"     : False,
   "safesync_url": "",
 }]
-target_os = ['linux']
+target_os = ['linux', 'android']