Compatibility work for ES 5.5.2

Author: mbok

README.adoc

@@ -171,7 +171,7 @@ matrix which matches your Elasticsearch version and use the download link for th
 
 [source]
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-./bin/elasticsearch-plugin install https://github.com/scaleborn/elasticsearch-linear-regression/releases/download/5.5.1.2/elasticsearch-linear-regression-5.5.1.2.zip
+./bin/elasticsearch-plugin install https://github.com/scaleborn/elasticsearch-linear-regression/releases/download/5.5.2.1/elasticsearch-linear-regression-5.5.2.1.zip
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 The plugin will be installed under the name "linear-regression".
 Do not forget to restart the node after installing.
@@ -180,6 +180,7 @@ Do not forget to restart the node after installing.
 [frame="all"]
 |===
 | Plugin version | Elasticsearch version | Release date
+| https://github.com/scaleborn/elasticsearch-linear-regression/releases/download/5.5.2.1/elasticsearch-linear-regression-5.5.2.1.zip[5.5.2.1]        | 5.5.2 | Aug  30, 2017
 | https://github.com/scaleborn/elasticsearch-linear-regression/releases/download/5.5.1.2/elasticsearch-linear-regression-5.5.1.2.zip[5.5.1.2]        | 5.5.1 | Aug  29, 2017
 | https://github.com/scaleborn/elasticsearch-linear-regression/releases/download/5.5.1.1/elasticsearch-linear-regression-5.5.1.1.zip[5.5.1.1]        | 5.5.1 | Jul  27, 2017
 | https://github.com/scaleborn/elasticsearch-linear-regression/releases/download/5.5.0.1/elasticsearch-linear-regression-5.5.0.1.zip[5.5.0.1]        | 5.5.0 | Jul  18, 2017