Update docker-api to be compatible with ruby 2.7

Updates the excon gem which was the one with ruby 2.7 problems

Gemfile.lock

@@ -122,9 +122,9 @@ GEM
     declarative (0.0.10)
     declarative-option (0.1.0)
     diff-lcs (1.3)
-    docker-api (1.33.1)
-      excon (>= 0.38.0)
-      json
+    docker-api (2.0.0)
+      excon (>= 0.47.0)
+      multi_json
     domain_name (0.5.20170404)
       unf (>= 0.0.5, < 1.0.0)
     ed25519 (1.2.4)
@@ -132,7 +132,7 @@ GEM
     equatable (0.6.1)
     erubi (1.9.0)
     erubis (2.7.0)
-    excon (0.54.0)
+    excon (0.76.0)
     fantaskspec (0.9.10)
       rake (~> 10.0)
       rspec (~> 3.0)