Remove Ubuntu 12.04 from Shippable CI.

Ubuntu 12.04 will be end-of-life before stable-2.4 is released.

shippable.yml

@@ -30,7 +30,6 @@ matrix:
     - env: TEST=linux/fedora25/1
     - env: TEST=linux/opensuse42.1/1
     - env: TEST=linux/opensuse42.2/1
-    - env: TEST=linux/ubuntu1204/1
     - env: TEST=linux/ubuntu1404/1
     - env: TEST=linux/ubuntu1604/1
     - env: TEST=linux/ubuntu1604py3/1
@@ -41,7 +40,6 @@ matrix:
     - env: TEST=linux/fedora25/2
     - env: TEST=linux/opensuse42.1/2
     - env: TEST=linux/opensuse42.2/2
-    - env: TEST=linux/ubuntu1204/2
     - env: TEST=linux/ubuntu1404/2
     - env: TEST=linux/ubuntu1604/2
     - env: TEST=linux/ubuntu1604py3/2
@@ -52,7 +50,6 @@ matrix:
     - env: TEST=linux/fedora25/3
     - env: TEST=linux/opensuse42.1/3
     - env: TEST=linux/opensuse42.2/3
-    - env: TEST=linux/ubuntu1204/3
     - env: TEST=linux/ubuntu1404/3
     - env: TEST=linux/ubuntu1604/3
     - env: TEST=linux/ubuntu1604py3/3