fix(build): properly reference rxjs from the UMD bundle
fix(build): properly reference rxjs from the UMD bundle

alpha.7 has a subtle bug where externals were not configured
properly and as a result our bundle doesn't work correctly
in certain environements (SystemJS).

Closes #884

fbasso/ng-bootstrap,johnbenz13/ng-bootstrap,spongessuck/ng-bootstrap,ng-bootstrap/core,jnizet/ng-bootstrap,ktriek/ng-bootstrap,Xaiid/ng-bootstrap,ktriek/ng-bootstrap,ng-bootstrap/ng-bootstrap,ng-bootstrap/ng-bootstrap,chenyuzhcy/ng-bootstrap,ng-bootstrap/ng-bootstrap,johnbenz13/ng-bootstrap,ACGC/ng-bootstrap,spongessuck/ng-bootstrap,deeg/ng-bootstrap,ktriek/ng-bootstrap,benouat/ng-bootstrap,Xaiid/ng-bootstrap,maxokorokov/core,Foxandxss/ng-bootstrap,CrustyJew/ng-bootstrap,johnbenz13/ng-bootstrap,Xaiid/ng-bootstrap,CrustyJew/ng-bootstrap,spongessuck/ng-bootstrap,deeg/ng-bootstrap,fbasso/ng-bootstrap,ng-bootstrap/core,Foxandxss/ng-bootstrap,maxokorokov/core,jnizet/ng-bootstrap,CrustyJew/ng-bootstrap,fbasso/ng-bootstrap,ng-bootstrap/ng-bootstrap,mschoudry/ng-bootstrap,ACGC/ng-bootstrap,maxokorokov/core,maxokorokov/core,CrustyJew/ng-bootstrap,ng-bootstrap/core,ktriek/ng-bootstrap,jnizet/ng-bootstrap,deeg/ng-bootstrap,benouat/ng-bootstrap,fbasso/ng-bootstrap,mschoudry/ng-bootstrap,pkozlowski-opensource/core,Foxandxss/ng-bootstrap,spongessuck/ng-bootstrap,jnizet/ng-bootstrap,benouat/ng-bootstrap,pkozlowski-opensource/core,pkozlowski-opensource/core,ACGC/ng-bootstrap,chenyuzhcy/ng-bootstrap,mschoudry/ng-bootstrap,deeg/ng-bootstrap,chenyuzhcy/ng-bootstrap,benouat/ng-bootstrap