changed minim symfony requirement to ~2.7.0
changed minim symfony requirement to ~2.7.0

There are already many usages in code that doesn't work on <2.6 symfony. I don't think is worth to try keep support for 2.3 version.

saimaz/ElasticsearchBundle,asev/ElasticsearchBundle,GrandLTU/ElasticsearchBundle,ongr-io/ElasticsearchBundle,trandangtri/ElasticsearchBundle,ndinh215/ONGRElasticsearchBundle,martiis/ElasticsearchBundle,saimaz/ElasticsearchBundle,abhiesa-tolexo/ElasticsearchBundle,murnieza/ElasticsearchBundle,tautrimas/ElasticsearchBundle,trandangtri/ElasticsearchBundle,ndinh215/ONGRElasticsearchBundle,murnieza/ElasticsearchBundle,abhiesa-tolexo/ElasticsearchBundle,martiis/ElasticsearchBundle,tautrimas/ElasticsearchBundle,GrandLTU/ElasticsearchBundle,asev/ElasticsearchBundle