Update .travis.yml to ajust to new release cadence (#148)

Also change uri parser regexp due to uri gem change

From ruby/uri#107:

```
This pull-request switched URI::DEFAULT_PARSER to URI::RFC3986_Parser. And I added switch-back feature for URI::RFC2396_Parser.

URI.parser = URI::RFC2396_PARSER
```

Introduce check so that if the `URI::RFC2396_PARSER` constant is not defined (using an older URI lib) then fall back to using the default.

Author: jsvd

.travis.yml

@@ -1,7 +1,10 @@
 import:
   - logstash-plugins/.ci:travis/[email protected]
 
-env:
-  jobs: # test with old scheduler version (3.0 was locked in LS 7.x)
-    - ELASTIC_STACK_VERSION=7.x RUFUS_SCHEDULER_VERSION=3.0.9 LOG_LEVEL=info
-    - ELASTIC_STACK_VERSION=8.2.0 RUFUS_SCHEDULER_VERSION=3.0.9
+jobs:
+  exclude:
+  - env: ELASTIC_STACK_VERSION=7.current
+  - env: SNAPSHOT=true ELASTIC_STACK_VERSION=7.current
+  include:
+  - env: ELASTIC_STACK_VERSION=7.current RUFUS_SCHEDULER_VERSION=3.0.9 SNAPSHOT=true
+  - env: ELASTIC_STACK_VERSION=7.current RUFUS_SCHEDULER_VERSION=3.0.9

lib/logstash/inputs/http_poller.rb

@@ -154,8 +154,8 @@ def normalize_request(url_or_spec)
   private
   def validate_request!(url_or_spec, request)
     method, url, spec = request
-
-    raise LogStash::ConfigurationError, "Invalid URL #{url}" unless URI::DEFAULT_PARSER.regexp[:ABS_URI].match(url)
+    parser = defined?(URI::RFC2396_PARSER) ? URI::RFC2396_PARSER : URI::DEFAULT_PARSER
+    raise LogStash::ConfigurationError, "Invalid URL #{url}" unless parser.regexp[:ABS_URI].match(url)
 
     raise LogStash::ConfigurationError, "No URL provided for request! #{url_or_spec}" unless url
     if spec && spec[:auth]