Put deprecation warning together in one line

[r7kamura]

Background

The existing deprecation warning contains a lot of funny whitespaces (maybe we should use <<~ instead of <<-), but I think the source of the problem is that the warning message is printed across multiple lines.

Changes

In this pull request, I'll put it together in one line by removing line breaks and all unnecessary spaces.

Before

This is a snippet of output from bundle exec rake:

.          [DEPRECATION] now assert_schema_conform check response schema only.
            but we will change check request and response in future major version.
            so if you want to conform response only, please use assert_response_schema_confirm,
            or you can suppress this message and keep old behavior by setting old_assert_behavior=true.
.          [DEPRECATION] now assert_schema_conform check response schema only.
            but we will change check request and response in future major version.
            so if you want to conform response only, please use assert_response_schema_confirm,
            or you can suppress this message and keep old behavior by setting old_assert_behavior=true.
...........................................................................

Finished in 3.028900s, 135.3627 runs/s, 263.1318 assertions/s.

After

.[DEPRECATION] now assert_schema_conform check response schema only. but we will change check request and response in future major version. so if you want to conform response only, please use assert_response_schema_confirm, or you can suppress this message and keep old behavior by setting old_assert_behavior=true.
.[DEPRECATION] now assert_schema_conform check response schema only. but we will change check request and response in future major version. so if you want to conform response only, please use assert_response_schema_confirm, or you can suppress this message and keep old behavior by setting old_assert_behavior=true.
................................................................................................................................

Finished in 3.032245s, 135.2134 runs/s, 262.8416 assertions/s.

[ota42y]

Nice changes!!!! thanks!!!