Add thumv8m.main-none-eabi to Travis test matrix

Author: thejpster

cortex-m-rt/.travis.yml

@@ -22,6 +22,10 @@ matrix:
       rust: stable
       if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
 
+    - env: TARGET=thumbv8m.main-none-eabi
+      rust: stable
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: nightly
       if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
@@ -42,6 +46,10 @@ matrix:
       rust: nightly
       if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
 
+    - env: TARGET=thumbv8m.main-none-eabi
+      rust: nightly
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
+
 before_install: set -e
 
 install: