Make pliny compatible with ruby 2.4

[cyberdelia]

This allow to run Pliny with ruby 2.4 by upgrading the activesupport gem, since it requires a version of the json gem that is incompatible with ruby 2.4.

Sadly this also means we have to drop support for ruby > 2.2.2 since activesupport do not support them.

[brandur]

@cyberdelia @gudmundur Do you know if the loss of 2.2.2 support is going to be a problem for anyone? It seems like Heroku's generally doing a pretty good job of keeping its Ruby stacks up-to-date.

[brandur]

@cyberdelia Mind fixing that one canonical line test this way instead?

diff --git a/spec/canonical_log_line_helpers_spec.rb b/spec/canonical_log_line_helpers_spec.rb
index 3509912..0290dec 100644
--- a/spec/canonical_log_line_helpers_spec.rb
+++ b/spec/canonical_log_line_helpers_spec.rb
@@ -22,9 +22,9 @@ describe Pliny::CanonicalLogLineHelpers do
   it "rejects values that are of the wrong type" do
     line = TestCanonicalLogLine.new
     e = assert_raises ArgumentError do
-      line.field_string = 42
+      line.field_string = true
     end
-    assert_equal "Expected field_string to be type String (was Fixnum)",
+    assert_equal "Expected field_string to be type String (was TrueClass)",
       e.message
   end

I think it's a little less janky because it's version agnostic. Thanks!

[cyberdelia]

@brandur I did just that.

[brandur]

Sweet! Well, as long as we're reasonably sure we're not going to leave too many people around by dropping 2.1, LGTM.

[gudmundur]

Did a quick search through the Heroku repos and most Ruby projects seem to be 2.3+. I'm bringing this in.