PYTHON-1830 Update to amazon linux 2018 for 4.2 support

Author: ShaneHarvey

.evergreen/config.yml

@@ -872,12 +872,10 @@ axes:
   - id: os-fully-featured
     display_name: OS
     values:
-      - id: linux-64-amzn-test
-        display_name: "Amazon Linux (Enterprise)"
-        run_on: linux-64-amzn-test
+      - id: amazon1-2018-test
+        display_name: "Amazon Linux 2018 (Enterprise)"
+        run_on: amazon1-2018-test
         batchtime: 10080  # 7 days
-        variables:
-           PYTHON_BINARY: "python2.7"
 
       - id: ubuntu-14.04
         display_name: "Ubuntu 14.04"