Move all 19.04-based jobs back to ubuntu 18.04

.drone.yml

@@ -8,7 +8,7 @@ platform:
 
 steps:
 - name: Build and Test
-  image: ubuntu:19.04
+  image: ubuntu:18.04
   environment:
     CC: gcc
     COMMON_FLAGS: 'DYNAMIC_ARCH=1 TARGET=ARMV8 NUM_THREADS=32'
@@ -32,7 +32,7 @@ platform:
 
 steps:
 - name: Build and Test
-  image: ubuntu:19.04
+  image: ubuntu:18.04
   environment:
     CC: gcc
     COMMON_FLAGS: 'DYNAMIC_ARCH=1 TARGET=ARMV6 NUM_THREADS=32'
@@ -152,7 +152,7 @@ platform:
 
 steps:
 - name: Build and Test
-  image: ubuntu:19.04
+  image: ubuntu:18.04
   environment:
     CC: gcc
     COMMON_FLAGS: 'USE_OPENMP=1'
@@ -176,7 +176,7 @@ platform:
 
 steps:
 - name: Build and Test
-  image: ubuntu:19.04
+  image: ubuntu:18.04
   environment:
     CC: gcc
     COMMON_FLAGS: 'USE_THREAD=1'