Add arm build for nightly flatpaks

.travis.yml

@@ -14,6 +14,7 @@ notifications:
 cache:
   directories:
     - .hunter
+    - build-flatpak/.flatpak-builder
 
 matrix:
     include:
@@ -103,7 +104,7 @@ matrix:
                     - libgl1-mesa-dev # needed for missing gl.h
         - os: linux
           env:
-              - DEPLOY=1
+              - DEPLOYMENT=1
               - FLATPAK=1
               - ARCH=amd64
           addons:
@@ -114,6 +115,21 @@ matrix:
                     - flatpak
                     - flatpak-builder
                     - elfutils
+        - os: linux
+          arch: arm64
+          env:
+              - DEPLOYMENT=1
+              - FLATPAK=1
+              - ARCH=arm64
+          addons:
+              apt:
+                  sources: 
+                    - sourceline: 'ppa:alexlarsson/flatpak'
+                  packages: 
+                    - flatpak
+                    - flatpak-builder
+                    - elfutils
+                    - librsvg2-bin
 
 before_install:
     # Use TRAVIS_TAG if defined, or the short commit SHA otherwise