Bump erlang, elixir and nerves_system_br CI versions

.circleci/config.yml

@@ -2,13 +2,13 @@ version: 2.0
 defaults: &defaults
   working_directory: /nerves/build
   docker:
-    - image: nervesproject/nerves_system_br:1.6.1
+    - image: nervesproject/nerves_system_br:1.7.2
 
 install_elixir: &install_elixir
   run:
     name: Install Elixir
     command: |
-      wget https://github.com/elixir-lang/elixir/releases/download/v1.8.0/Precompiled.zip
+      wget https://github.com/elixir-lang/elixir/releases/download/v1.8.1/Precompiled.zip
       unzip -d /usr/local/elixir Precompiled.zip
       echo 'export PATH=/usr/local/elixir/bin:$PATH' >> $BASH_ENV
 

.tool-versions

@@ -1,2 +1,2 @@
-erlang 21.3.6
+erlang 21.3.7
 elixir 1.8.1-otp-21