angularspree merge[git submodule] (#397)

### Why?
AngularSpree is now a part of Aviacommerce project, we need to keep both(front-end, backend) the repos in one single repo.

This will result in better experience while setting up this project locally.

### This change addresses the need by?
Adding a submodule for AngularSpree repo in the avia repo.

We'll update angularspree remote every 15 days.
Primary development will happen in this repository.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "front-end"]
+	path = front-end
+	url = [email protected]:aviabird/angularspree.git

.travis.yml

@@ -3,6 +3,9 @@ language: elixir
 otp_release:
   - 20.2
 before_install:
+  # Use sed to replace the SSH URL with the public URL, then initialize submodules
+  - sed -i 's/[email protected]:/https:\/\/github.com\//' .gitmodules
+  - git submodule update --init --recursive
   - mix local.hex --force
   - mix local.rebar --force
   - mix deps.get
@@ -40,5 +43,9 @@ env:
 notifications:
   email:
     recipients:
-      - ananya95+travis@gmail.com
+      - developers@aviabird.com
 
+# Handle git submodules yourself
+git:
+  submodules: false
+