Upgrade Esper version

[fgalan]

Currently we are using

<dependency>
            <groupId>com.espertech</groupId>
            <artifactId>esper</artifactId>
            <version>4.7.0</version>
</dependency>

This seems to be pretty old. From http://www.espertech.com/esper/esper-documentation, current version (in the moment of writting this) is 7.0.0 and the older supported one seems to be 4.10.0 (dated 2013).

We have to upgrade to a newer version

[fgalan]

From http://www.espertech.com/esper/esper-changehistory, last version allowsing Java 7 is 5.5.0. Next inmediate version (6.0.1) needs Java 8:

Requires Java 8

In fact, trying mvn test with that version results in:

Caused by: java.lang.UnsupportedClassVersionError: com/espertech/esper/client/EventBean : Unsupported major.minor version 52.0

(52.0 is the "code" for Java 8, see https://stackoverflow.com/questions/22489398/unsupported-major-minor-version-52-0)

We could go to a newer Esper version but changing Perseo Core requierements to Java 8 (impacting documentation and dockerfile, at least).

[fgalan]

Esper version upgraded to 5.5.0 in PR #88. We have to decide if we stop there or go further through an upgrade of the Java version to Java 8.

[fgalan]

Esper verseion upgrade to 6.1.0, given that Perseo Core Java version were upgrade to Java 8 in the last months.

There is a Esper 7.1.0 version in the moment of writting this (http://www.espertech.com/esper/esper-changehistory) but the sentence "tested against Java 9" is a bit scaring. Thus, I think is ok by the moment with 6.1.0 and this issue can be closed.