test on Go 1.5 on Travis CI and update docs for the CalcCellValue

.travis.yml

@@ -8,6 +8,7 @@ go:
   - 1.12.x
   - 1.13.x
   - 1.14.x
+  - 1.15.x
 
 os:
   - linux

calc.go

@@ -96,6 +96,19 @@ type formulaFuncs struct{}
 // CalcCellValue provides a function to get calculated cell value. This
 // feature is currently in working processing. Array formula, table formula
 // and some other formulas are not supported currently.
+//
+// Supported formulas:
+//
+//    ABS, ACOS, ACOSH, ACOT, ACOTH, ARABIC, ASIN, ASINH, ATAN2, ATANH, BASE,
+//    CEILING, CEILING.MATH, CEILING.PRECISE, COMBIN, COMBINA, COS, COSH, COT,
+//    COTH, COUNTA, CSC, CSCH, DECIMAL, DEGREES, EVEN, EXP, FACT, FACTDOUBLE,
+//    FLOOR, FLOOR.MATH, FLOOR.PRECISE, GCD, INT, ISBLANK, ISERR, ISERROR,
+//    ISEVEN, ISNA, ISNONTEXT, ISNUMBER, ISO.CEILING, ISODD, LCM, LN, LOG,
+//    LOG10, MDETERM, MEDIAN, MOD, MROUND, MULTINOMIAL, MUNIT, NA, ODD, PI,
+//    POWER, PRODUCT, QUOTIENT, RADIANS, RAND, RANDBETWEEN, ROUND, ROUNDDOWN,
+//    ROUNDUP, SEC, SECH, SIGN, SIN, SINH, SQRT, SQRTPI, SUM, SUMIF, SUMSQ,
+//    TAN, TANH, TRUNC
+//
 func (f *File) CalcCellValue(sheet, cell string) (result string, err error) {
 	var (
 		formula string