Add rpm to Linux build outputs.

- Explicitly defining `depends` for the defaults is not necessary as of
  electron-userland/electron-builder#502

.travis.yml

@@ -29,6 +29,7 @@ before_install:
       sudo apt-get update -qq;
       sudo apt-get install -qq libxml2-dev;
       sudo apt-get install -qq libappindicator1;
+      sudo apt-get install -qq rpm;
     fi
 install:
   - git config --global core.autocrlf input

package.json

@@ -91,13 +91,10 @@
 			"target": [
 				"AppImage",
 			        "deb",
+                                "rpm",
 				"zip",
 				"tar.gz"
 			],
-			"depends": [
-				"libappindicator1",
-				"libnotify-bin"
-			],
 			"icon": "./resources/Icon.png"
 		}
 	},