Use latest release build of ffmpeg for mac build

Also added gpg verification of the file since it was available

.travis.yml

@@ -57,8 +57,7 @@ before_install:
         if [ "$ver" -le "27" ]; then
             brew upgrade python;
         fi
-        brew install p7zip;
-        brew install qt;
+        brew install p7zip qt gpg;
         brew link qt --force;
     fi
 
@@ -120,9 +119,13 @@ after_success:
 
       echo "Copying ffmpeg plugin";
       mkdir Pencil2D.app/Contents/MacOS/plugins;
-      wget -P Pencil2D.app/Contents/MacOS/plugins https://evermeet.cx/pub/ffmpeg/ffmpeg-3.4.2.7z;
-      7z x Pencil2D.app/Contents/MacOS/plugins/ffmpeg-3.4.2.7z -o"Pencil2D.app/Contents/MacOS/plugins";
-      rm Pencil2D.app/Contents/MacOs/plugins/ffmpeg-3.4.2.7z;
+      wget -O ffmpeg.7z https://evermeet.cx/ffmpeg/getrelease/7z;
+      wget -O ffmpeg.7z.sig https://evermeet.cx/ffmpeg/getrelease/7z/sig;
+      echo "trusted-key 0x476C4B611A660874" > ~/.gnupg/gpg.conf;
+      curl https://evermeet.cx/ffmpeg/0x1A660874.asc | gpg --import;
+      gpg --verify ffmpeg.7z.sig ffmpeg.7z;
+      7z x ffmpeg.7z -o"Pencil2D.app/Contents/MacOS/plugins";
+      rm ffmpeg.7z ffmpeg.7z.sig;
 
       echo "Copying necessary Qt frameworks";
       macdeployqt Pencil2D.app;