feat: update to angular 6, RXJs 6 (ng-select#545)

* feat: update to angular 6, RXJs 6
BREAKING CHANGE: this changes utilizes new RXJs 6 which is not with previous version. Due to that angular version was also.
closes ng-select#530

ng-select/ng-dropdown-panel.component.ts

@@ -25,9 +25,8 @@ import { NgSelectComponent, DropdownPosition } from './ng-select.component';
 import { ItemsList } from './items-list';
 import { WindowService } from './window.service';
 import { VirtualScrollService } from './virtual-scroll.service';
-import { fromEventPattern } from 'rxjs/observable/fromEventPattern';
 import { takeUntil } from 'rxjs/operators';
-import { Subject } from 'rxjs/Subject';
+import { Subject, fromEventPattern } from 'rxjs';
 
 const TOP_CSS_CLASS = 'ng-select-top';
 const BOTTOM_CSS_CLASS = 'ng-select-bottom';

ng-select/ng-option.component.ts

@@ -6,7 +6,7 @@ import {
     OnChanges,
     SimpleChanges
 } from '@angular/core';
-import { Subject } from 'rxjs/Subject';
+import { Subject } from 'rxjs';
 
 @Component({
     selector: 'ng-option',

ng-select/ng-select.component.spec.ts

@@ -16,7 +16,7 @@ import { FormsModule } from '@angular/forms';
 import { NgSelectModule } from './ng-select.module';
 import { NgSelectComponent } from './ng-select.component';
 import { KeyCode, NgOption } from './ng-select.types';
-import { Subject } from 'rxjs/Subject';
+import { Subject } from 'rxjs';
 import { WindowService } from './window.service';
 import { TestsErrorHandler, tickAndDetectChanges, triggerKeyDownEvent, getNgSelectElement, selectOption } from '../testing/helpers';
 import { MockNgZone, MockNgWindow, MockConsole } from '../testing/mocks';

ng-select/ng-select.component.ts

@@ -26,8 +26,7 @@ import {
 } from '@angular/core';
 import { ControlValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';
 import { takeUntil, startWith, tap, debounceTime, map, filter } from 'rxjs/operators';
-import { merge } from 'rxjs/observable/merge';
-import { Subject } from 'rxjs/Subject';
+import { Subject, merge } from 'rxjs';
 
 import {
     NgOptionTemplateDirective,

package.json

@@ -21,9 +21,9 @@
     "angular4"
   ],
   "peerDependencies": {
-    "@angular/common": ">=5.0.0 <7.0.0",
-    "@angular/core": ">=5.0.0 <7.0.0",
-    "@angular/forms": ">=5.0.0 <7.0.0"
+    "@angular/common": ">=6.0.0 <7.0.0",
+    "@angular/core": ">=6.0.0 <7.0.0",
+    "@angular/forms": ">=6.0.0 <7.0.0"
   },
   "ngPackage": {
     "lib": {