ELFunctionDefinition do not work because of Guava

[ylacaute]

Jinjava : 2.5.7
Java : 8

There are 2 problems to use ELFunctionDefinition.

1) Guava
The Guava lib coming from Jinjava 2.5.7 is 19, but it generates a Guava class exception. Adding Guava 21 solve the problem.

2) Namespace not supported
Namespaces are still not supported, the example in the README is wrong. A workaround is just to set an empty String for the namespace.

[chrisirhc]

Looks like there's a usage of toImmutableMap which requires guava 21.

jinjava/src/main/java/com/hubspot/jinjava/lib/filter/AbstractFilter.java

Line 249 in 9f9d5c8

ImmutableMap.toImmutableMap(Map.Entry::getKey, e -> e.getValue().defaultValue())

Perhaps the pom.xml should be updated to enforce this requirement.