bump x.y.z versions to even y prior to creation of RELEASE_3_6 branch

DESCRIPTION

 Package: RMassBank
 Type: Package
 Title: Workflow to process tandem MS files and build MassBank records
-Version: 2.5.1
+Version: 2.6.0
 Authors@R: c(
     person(given = "RMassBank at Eawag", email = "massbank@eawag.ch",
     role=c("cre")),