RUBY-2058 test driver on Ruby 2.7 (#1615)

Author: p-mongo

.evergreen/config.yml

@@ -618,6 +618,10 @@ axes:
         display_name: ruby-2.6
         variables:
            RVM_RUBY: "ruby-2.6"
+      - id: "ruby-2.7"
+        display_name: ruby-2.7
+        variables:
+           RVM_RUBY: "ruby-2.7"
       - id: "ruby-head"
         display_name: ruby-head
         variables:
@@ -736,7 +740,7 @@ buildvariants:
   matrix_name: "mongo"
   matrix_spec:
     auth-and-ssl: ["auth-and-ssl", "noauth-and-nossl"]
-    ruby: "ruby-2.6"
+    ruby: "ruby-2.7"
     mongodb-version: "*"
     topology: "*"
   display_name: "${mongodb-version} ${topology} ${auth-and-ssl} ${ruby}"

.evergreen/functions.sh

@@ -95,8 +95,8 @@ setup_ruby() {
     if ! test "$RVM_RUBY" = ruby-1.9; then
 
     # For testing toolchains:
-    toolchain_url=https://s3.amazonaws.com//mciuploads/mongo-ruby-toolchain/`host_arch`/741a9eba5788e6d5254a09319d7d237b40a9f4d6/mongo_ruby_driver_toolchain_`host_arch`_patch_741a9eba5788e6d5254a09319d7d237b40a9f4d6_5cfa8ad0d6d80a02bbb45be7_19_06_07_16_03_30.tar.gz
-    curl -fL $toolchain_url |tar zxf -
+    toolchain_url=https://s3.amazonaws.com//mciuploads/mongo-ruby-toolchain/`host_arch`/e7cf68d7146c09d54dfbe241c04aad3e3eadbb10/mongo_ruby_driver_toolchain_`host_arch |tr - _`_e7cf68d7146c09d54dfbe241c04aad3e3eadbb10_19_12_27_00_47_13.tar.gz
+    curl --retry 3 -fL $toolchain_url |tar zxf -
     export PATH=`pwd`/rubies/$RVM_RUBY/bin:$PATH
 
     # Attempt to get bundler to report all errors - so far unsuccessful